Working with Big Integers in PHP

<?php
/**
* @author Erik Giberti
* @package Big Integer Math
*/
 
class bigint {
 
	public static function base_convert($value_in = false, $source_base = 10, $target_base = 36){
		// We use these values for our conversions
		$values = array('0','1','2','3','4','5','6','7','8','9','a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z','A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z','-','_');
		$characters = array_flip($values);
		$string_out = '';
 
		// Easy answers
		if($source_base	== $target_base){ return $value_in; }
 
		$len = strlen($value_in);
		$sum = array();
		if($source_base > $target_base){
			for($i=0;$i<$len;$i++){
				$char = substr($value_in, $len - $i - 1, 1);
				$sum[$i] = $characters[$char] * pow($source_base, $i);
			}
		} else {
			if($source_base != 10){
				$value = bigint::base_convert($value_in, $source_base, 10);
			} else {
				$value = $value_in;
			}
 
			$cnt = 0;
			while($value > 0){
				$times = $value / $target_base;
				$sum[] = bigint::mod($value, $target_base);
				$value = bigint::floor($times); // get rid of our remainder
			}
 
			for($i=count($sum);$i>0;$i--){
				$string_out .= $values[$sum[$i-1]];
			}
 
			return $string_out;
		}
 
		if($target_base == 10){ return array_sum($sum); }
		return implode($sum);
	}
 
	public static function mod($val, $mod){ return $val - floor($val/$mod) * $mod; }
 
	public static function floor($val){
		$bits = explode('.',$val);
		return $bits[0];
	}
}